"In situ-like" mantle cell lymphoma: a report of two cases.

Mantle cell lymphoma (MCL) is a B cell neoplasm that most often shows a diffuse growth pattern. Two cases of MCL are reported here, both with a previous diagnosis of lymphoid hyperplasia. Morphologically, germinal centres are hyperplasic with a normal or discretely enlarged mantle zone, where foci of irregularly shaped small lymphocytes are seen. These are positive for CD20, CD5 and cyclin D1, confirming a diagnosis of in situ-like MCL. This type differs from the mantle zone pattern in that the neoplastic mantle zone is very thin and there is very little or no spread of tumour cells into interfollicular areas. To the best of our knowledge, this is the first report on such a pattern of MCL, which is important to recognise, as it can be confused with lymphoid hyperplasia.